A conventional method of producing tailored fiber placement (TFP) structures is to sew reinforcing fibers onto a substrate. However, the thickness of an individual TFP structure is limited to approximately 5 mm because sewing additional reinforcing fibers onto the structure already formed damages the fibers previously sewn on. In addition, the sewing threads used previously to fix the reinforcing fibers, as well as the sewing substrate and the under-threads accumulated on the bottom of the substrate material remain in the TFP structure. When multilayer TFP structures made up of more than two individual TFP structures are produced, at least one of these layers containing high amounts of fixing yarn remains within the structure in addition to the fixing threads.
Previously, it was possible to form a preform without an intermediate layer from a maximum of two TFP structures, the two individual TFP structures being joined together in such a way that the substrate and under-thread accumulation are always located on the outside of the formed structure. At the same time, this means that such a preform formed from two individual TFP structures has a maximum thickness of approximately 10 mm. However, in this case also, the sewing threads are still contained in the fiber composite part after impregnation and curing and represent imperfections in the material. The constrictions and displacements of the reinforcing fibers caused by the fixing threads and the usually poor connection of the fixing threads to the matrix material have an adverse effect on the mechanical properties of the material.
A moldable, multiaxial reinforcing structure is described in European Published Patent Application No. 0 567 845, it being possible to position the reinforcing fibers in any direction appropriate to the stress conditions using embroidery technology. The glass transition temperature (or softening point) of the embroidery yarns is in this case above the softening point of the composite material in order to ensure a secure fixing of the reinforcing fibers within the composite until the final molding.
In addition, German Published Patent Application No. 196 28 388 describes a force flux-appropriate, multiaxial, multilayer fiber preform having Z-axis reinforcement at least in some areas and a method of manufacturing same. Z-axis reinforcing fibers are incorporated by embroidery at least in some areas to accommodate the force flux in the Z-axis.
It is an object of the present invention to provide a method making it possible to produce multilayer TFP preforms of any thickness and without interference by fixing threads or intermediate layers in a simple manner.